<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* li:nth-child(odd){
            color: orchid;
        }
        li:nth-child(even){
            color: orangered;
            
        } */
    </style>
</head>

<body>
    <ul>
        <li>0</li>
        <li>1</li>
        <li>2</li>
        <li>3</li>
        <li>4</li>
        <li>5</li>
        <li>6</li>
        <li>7</li>
        <button id="even">even</button>
        <button id="odd">odd</button>
    </ul>
    <script>
        var lis = document.getElementsByTagName("li");
        var even = document.getElementById("even");
        var odd = document.getElementById("odd");
        even.onclick = function () {
            for (var i = 0; i < lis.length; i++) {
                if (i % 2 == 0) {
                    lis[i].style.color = "red"
                }
            }
        }
        odd.onclick = function () {
            for (var i = 0; i < lis.length; i++) {
                if (i % 2 != 0) {
                    lis[i].style.color = "green"
                }
            }
        }
    </script>
</body>

</html>